I think I would go with the majority here and say concealer! I mean, I leave the house without makeup all the time, including - GASP! - going to work barefaced. A lot less lately though...I'm hoping that it's not only because I'm new to makeup. I hate to think of a day coming where I don't want to go out without it!
That said, even on days I don't feel like putting in the effort, about 80% of the time I dab a little concealer on my undereye circles. Just makes me feel fresher and more awake. Now, if you counted Chapstick I'd say that, since I have to have a tube in my pocket or purse AT ALL TIMES...but that's really a skincare thing. It's not remotely to do with vanity, but rather comfort!